The overall objective is to correlate the structures of growth hormone and prolactin to their biological activities. A more immediate goal is to determine the structure and physiological significance of the newly recognized forms of human growth hormone that can be isolated from pituitary extracts. These forms appear to be of three types: a) structural variants of the 191 amino acid form with molecular weights of approximately 22,000; b) enzymically altered forms with from 6-15 amino acids cleaved from the large disulfide loop; and c) fragments of the hormone with molecular weights as small as 5,000. There is evidence that the different forms and fragments have varying biological and immunological activities. The methods used to carry out the purifications are standard protein isolation techniques. The reason for these forms having been overlooked is that they tend to aggregate as a complex which has been termed "human growth hormone". Only when dissociating media, such as urea and SDS buffers, are used during isolation do the forms separate. Electrophoretic techniques are capable of dissociated the complex and these are used to monitor the separations. Structure work is done by Edman automatic and manual sequencing, combined with peptide mapping. Standard bioassays in the rat are used for measurement of growth promoting activity; the pigeon is used for assay of lactogenic properties and the dog for determining diabetogenic activity. Radioimmunoassays are used to evaluate immunological characteristics. From these preliminary studies we have developed the hypothesis that human growth hormone, as found in the pituitary gland, is a complex of structurally similar proteins and each of these is a prohormone which must be activated proteolytically to an active form.